1. Field of the Invention
This invention relates to a light source optical system for endoscopes, particularly having an elliptical condensing mirror projecting a bright spot of the light-emitting section of a lamp and a relay optical system transmitting the projected image of the bright spot to the entrance end face of a light guide.
2. Description of Related Art
In general, observation with an endoscope needs an illumination system including, at least, a light source supplying light to illuminate a subject for observation and a light guide transmitting the light emitted from the light source to the distal end of the endoscope. The light source is constructed with a discharge lamp giving out intense light, such as a xenon lamp or a metal halide lamp, and a condensing optical system efficiently collecting the light emitted from this lamp on the entrance end face of the light guide. As an example, a light source optical system set forth in Japanese Utility Model Preliminary Publication No. Hei 1-135408 is known. This optical system includes an elliptical condensing mirror placed so that the center of the light-emitting section of the discharge lamp is located at the primary focal point thereof, and first and second condenser lenses situated behind the secondary focal point of the elliptical condensing mirror to collect light. The optical system has the function that the bright spot of the light-emitting section is projected in a space by the elliptical condensing mirror and the projected image is transmitted to the entrance end face of the light guide by the condenser lenses.
Recently, by the widespread use of endoscopes, their applications to observations have been diversified, and endoscope observation systems with high versatility which can accommodate such applications have been in demand. In keeping with this, the improvements of an observer's work efficiency and of ease with which the observer handles apparatuses have come into big problems to be solved. Endoscopes are available in different kinds, such as an endoscope of relatively large diameter for observing and treating the stomach or intestines and an endoscope of extremely small diameter for observing the interior of a blood vessel. Such endoscopes require light source apparatuses to supply illumination light with brightness sufficient for such observations. In view of the observer's work efficiency, it is imperative to provide a light source apparatus with lightweight and compact design such that it is easy to carry and does not occupy much space when placed.
In the light source optical system of this type, on the other hand, the light-emitting section of the lamp has a light-emitting area with a certain size, and thus a question arises as to how efficiently the light from the lamp is collected on the entrance end face of the light guide. Specifically, the question is due to not only how the focal length of the elliptical condensing mirror is determined with respect to the light-emitting section having a certain dimension along the optical axis, but also how the effective aperture diameter of the elliptical condensing mirror having the determined focal length is determined to optimize the brightness of the light collected on the end face of the light guide and the size of the light beam.